It is a question of protecting the optical film or film multilayer from attack due to the operating environment: moisture, pollution, heat, etc., by superposing and adhesively bonding a transparent protective cover that is substantially the same size as the transparent substrate to which the film or film multilayer is adhesively bonded, the film or film multilayer being set back from the edge. The encapsulation has the technical effect of preserving the optical properties of these one or more films over time. It furthermore allows the device to be reinforced with a cover that may be relatively thick and relatively stiff. This effect is commonly obtained by enclosing the optical film or film multilayer between the substrate and a protective cover of corresponding dimensions. The cover is made of an optically transparent material, for example of glass. It is adhesively bonded on top of the substrate and the film or film multilayer using a layer of transparent adhesive having a suitable refractive index. This adhesive layer covers the surface of the film or film multilayer, and its edge face around its entire circumference. The edge face of the film or films is thus protected from moisture by the adhesive, which forms a seal, and the protective cover above, which furthermore acts to strengthen the structure. By way of example, a liquid-crystal display, also called an LCD, is described hereafter. The basic structure of such a display is illustrated in FIG. 1. It comprises a liquid-crystal layer 3 enclosed between two transparent substrates 1 and 2, and a polarizing optical film on the external face of each substrate. The back side, though which the display receives the light flux, which will be modulated by way of suitable electrical control of the liquid-crystal layer, is formed by the back substrate 1 and by the polarizing film 9 placed on the external face of the substrate, set back from the edge. The front side, on which a resulting image will form, comprises the front substrate 2 and the polarizing film 4 placed on the external face of the substrate, set back from the edge.
In the example, a protective cover 6 is adhesively bonded to the front side by means of an adhesive layer 5 that covers the optical film 4 and the parts 2ZP of the front substrate exceeding the optical film 4.
To adhesively bond the protective cover 6 to the front side of such a display comprising an optical film 4 set back from the edge of the front substrate 2, two methods may be used.
A first method employs an adhesive to laminate the protective cover 6 to the front side. However this method assumes that hot-melt lamination is possible. This is because it is necessary to melt the adhesive in order to completely fill the space between the front substrate 2 and the cover 6 in the parts 2ZP of the substrate that exceed the optical film. It is furthermore necessary to carry out this hot-melt lamination under vacuum so as to prevent air bubbles from being trapped under the cover. This method thus requires complex and therefore expensive equipment.
Moreover, laminating adhesives in practice take the form of thin strips typically about 50 μm in thickness, whereas the optical film is closer to about 250 μm in thickness. To sufficiently relax the mechanical stresses on the display, and to obtain a good coverage of the edge face of the optical film, it is necessary to laminate a number of layers of adhesive in succession. Thus this method, which at first glance would appear to be advantageous because it is simple to implement, in practice turns out to be expensive.
Another method employs deposition of an uncured adhesive of suitable viscosity. This method has the advantage of being implemented at room temperature and atmospheric pressure. It provides a uniform joint between the cover and the display, the interface with the optical film being free from bubbles.
However, this method has a major drawback: the curing, or polymerization, of the adhesive, conventionally carried out under UV or with heat, causes the adhesive to shrink in every direction and notably therefore in the thickness direction. This shrinkage may be about 10%. On account of the small thicknesses concerned (relative to the dimensions of the area of the front side) and of the step difference between the optical film and the substrate to which the optical film is adhesively bonded, the shrinkage of adhesive in the thickness direction is critical and leads to mechanical stresses that are liable to damage the product.
To illustrate this problem, consider, as indicated in FIG. 2, which is a partial view of the schematic cross section in FIG. 1, a 250 μm-thick optical film. Assuming the deposition of adhesive is carried out so as to obtain a final adhesive thickness of about 200 μm on the optical film, the thickness of adhesive around the optical film, between the parts 2ZP of the substrate that exceed the optical film and the cover 6, must be about 450 μm.
Above the optical film, curing causes the adhesive to shrink 20 μm, i.e. 10% of 200 μm.
Above the substrate, in the parts 2ZP exceeding the optical film, the adhesive shrinks 45 μm, i.e. 10% of 450 μm.
There is therefore a difference in the adhesive thickness between the active zone above the optical film and the peripheral zone, which difference is about 25 μm.
In practice, depending on the thickness of adhesive, the stiffness of the display, the material of the protective screen and its thickness, the mechanical stresses, which will naturally act to compensate this difference, may have various effects, which may be combined, notably:                strain in the display, this strain then leading to a variation in the thickness of the liquid crystal at the periphery of the active zone, degrading the appearance of the product;        delamination of the encapsulation at the adhesive/substrate and/or adhesive/cover interface, thus making it easier for moisture to penetrate and therefore adversely affecting the reliability of the final product; and        introduction of localized bubbles at the periphery of the optical film, degrading the appearance of the product or even its reliability if these bubbles reach the edge of the substrate, which may notably be the case when the distance between the edge of the optical film and the edge of the substrate is small.        
Thus, techniques for adhesively bonding a protective cover to a face of an electro-optical device, which face is not perfectly flat but comprises a height difference due to the presence of an optical film or film multilayer that is adhesively bonded to a substrate but set back from the edge, cause mechanical stresses which affect the quality of the finished product.